1. Field of the Invention
The present invention relates to a liquid crystal display, or in particular to an optical alignment layer used for the liquid crystal display.
2. Description of the Related Art
Normally, in the display operation of the liquid crystal display, the electric field is applied to the liquid crystal particles of the liquid crystal layer held between a pair of substrates, thereby changing the direction of alignment of the liquid crystal molecules and the optical characteristics of the liquid crystal layer. The conventional liquid crystal display of what is called active drive type comprising a switching element such as a thin-film transistor for each pixel is represented by the twisted nematic (TN) display system using the rotary polarization of the liquid crystal particles making up the liquid crystal layer, in which a pair of the substrates holding the liquid crystal layer each have an electrode and the electric field is applied to the liquid crystal layer in the direction substantially perpendicular to the substrate boundary. The greatest problem of the liquid crystal display of TN type is considered a narrow angular field of view.
On the other hand, JP-B-63-21907, U.S. Pat. No. 4,345,249, WO91/10936, JP-A-6-22739 and JP-A-6-160878 disclose an IPS system in which the electric field generated using the interdigital electrode formed on one of a pair of substrates has a component substantially parallel to the particular substrate and the liquid crystal particles making up the liquid crystal layer are rotated in a plane substantially parallel to the substrates to perform the display operation using the double refraction of the liquid crystal layer. The IPS system has the advantage of a wider angular field of view and a smaller load capacity than the conventional TN system due to the in-plane switching of the liquid crystal particles. The IPS system has thus seen a remarkable development recently as a promising candidate for a new liquid crystal display replacing the TN system. Also, JP-A-9-73101 discloses another IPS system with an improved transmittance in which at least one of the two electrodes for applying the electric field to the liquid crystal layer is configured of a transparent conductive film.
The liquid crystal display of IPS type (hereinafter referred to as IPS-TFT-LCD) having a superior visual angle characteristic (brightness contrast ratio, gradation and color tone inversion) with a high display brightness is a promising technique for the monitor and the TV receiver having a large display area. In the liquid crystal display, an alignment layer having the ability to align the liquid crystal particles is formed in the boundary between the liquid crystal layer and each of the two substrates holding the particular liquid crystal layer. For future practical applications of the IPS-TFT-LCD for the screen of 20 inches or larger, however, the development of a new structure and a new fabrication process suitable for a large-sized display (large-sized display panel) is required.
Especially, in the IPS-TFT-LCD having many steps on the substrate surface facing the liquid crystal layer, it is difficult to process the alignment layers to secure a uniform alignment over a large screen. The margin allowed for the alignment process executed for the alignment layer is very small as compared with the conventional TN system, or especially the normally-white TN system (bright image at low voltage, and dark image at high voltage) which is the current main stay. The reasons for the small margin are three points (1) to (3) described below.
(1) Stepped Structure
In IPS-TFT-LCD, a multiplicity of elongate electrodes (hereinafter sometimes referred to as the interdigital electrodes) having a width of about several μm are basically required to be arranged. A fine stepped structure is thus formed. The size of each step, depending on the thickness of the electrodes and the shape of the various films formed on the electrodes, is normally not less than 100 nm. In the high apertures pixel structure, a considerably thick organic insulating film is formed and the step roughness of and under the organic insulating film is flattened. Thus, the step of each alignment layer having a high aperture pixel structure is attributable mainly to the electrodes above the organic insulating film.
An alignment layer (also called the “alignment control layer”) composed of a high polymer film of polyimide or the like is formed as the uppermost ones of these layers.
In the conventional mass production technique, each alignment layer is rubbed to create the ability to align the liquid crystal particles (initial alignment). The rubbing cloth is composed of a bundle of thin fibers each about 10 to 30 μm thick. The ability to align the liquid crystal particles is created substantially by the shearing force generated in a predetermined direction by each thin fiber at local portions of the alignment layer. Although a microfiber about several microns thick is available, the requirement of the rigidity to create some friction force for the rubbing application eliminates the practical applications of the microfiber. In the IPS system, the interval between the electrodes is about 10 to 30 μm and substantially equal to the diameter of the above-mentioned fiber. Therefore, in the neighborhood of each step, the rubbing is liable to be insufficient and the alignment is easily disrupted. This alignment irregularity gives rise to an increased black level with the resulting lower contrast ratio, an uneven brightness or other image quality deterioration.
(2) Orientation Angle
In IPS-TFT-LCD, the direction of initial alignment is basically required to be set at an angle displaced at least a predetermined angle from the extension of the electrodes or the direction perpendicular to the particular extension. The electrode herein is defined as a signal wiring electrode, a common electrode in the pixel or a source electrode. To define the direction of the initial alignment by rubbing, the fiber about 10 to 30 μm in size is required to be used for rubbing at a predetermined angle as described above. The wiring extending in a predetermined direction and the steps at the ends of the wiring for the signal wiring electrode, the common electrode in the pixel and the source electrode draw the fiber from a set angle to the direction of the steps and disrupts the alignment, thereby increasing the black level or otherwise deteriorating the image quality.
(3) Settlement of Black Level
One of the features of the IPS-TFT-LCD is a superior settlement of the black level (black image). As compared with other systems, therefore, the disruption of the alignment is often conspicuous. In the conventional normally white TN system, the dark level is obtained with a high voltage applied. At a high voltage, most of the liquid crystal particles are aligned in one direction of the electric field perpendicular to the substrate surface, and the dark level is obtained from the relative positions of the liquid crystal particles and the polarizing plate. The uniformity of the dark level, therefore, is not basically dependent on the initial alignment at a low voltage. Further, the human eyes recognize the brightness irregularities as a relative ratio of brightness and reacts to them almost in logarithmic scale. The human eyes, therefore, are very sensitive to the change in dark level. Also from this viewpoint, the conventional normally white TN system in which the liquid crystal particles are aligned forcibly in one direction at a high voltage makes less sensitive to the initial alignment and therefore advantageous.
The IPS system, on the other hand, in which the dark level is displayed at low or zero voltage, is more sensitive to the disruption of the initial alignment. Especially, in the configuration (called double refraction) in which the liquid crystal particles are aligned homogeneously in parallel to each other on the upper and lower substrates and the light transmission axis of one of the polarizing plates is arranged in parallel and the other polarizing plate at right angles to the direction in which the liquid crystal particles are aligned, the polarized light incident to the liquid crystal layer is propagated substantially without disruption of the linear polarization. This effectively settles the dark level.
The transmittance T of the double refraction mode is generally expressed by the equation below.T=T0·sin2 {2α(E)}·sin2 {(π·deff·Δn)/λ}where T0 is a coefficient determined mainly by the transmittance of the polarizing plate used with the liquid crystal panel, α(E) the angle between the direction of alignment of the liquid crystal particles (effective optical axis of the liquid crystal layer) and the polarized light transmission axis, E the applied electric field strength, deff the effective thickness of the liquid crystal layer, Δn the anisotropic index of refraction of the liquid crystal, and λ the wavelength of the light. Also, the product deff·Δn of the effective thickness deff of the liquid crystal layer and the anisotropic index of refraction Δn of the liquid crystal is called the “retardation”. The thickness deff of the liquid crystal layer is not that of the whole liquid crystal layer, but corresponds to the thickness of the liquid crystal layer in which the alignment of the liquid crystal particles is actually changed upon application of a voltage thereto. This is by reason of the fact that the liquid crystal particles in the neighborhood of the boundary of the liquid crystal layer fail to change the direction of alignment thereof even upon application of a voltage thereto due to the “anchoring” effect on the boundary surface. Let dLC be the thickness of the whole liquid crystal layer held by the substrates. Then, the relation deff<dLC always holds between the thickness dLC and deff. The difference between deff and dLC, depending on the liquid crystal material used for the liquid crystal panel, the type of the material of the boundary in contact with the liquid crystal layer such as the type of the alignment layer material, is estimated at about 20 nm to 40 nm.
As apparent from the equation above, the term sin2 {2α(E)} is dependent on the field strength, and the brightness can be adjusted by changing the angle α in accordance with the field strength E. The normally black mode is achieved by setting the polarizing plate at α=0 with no voltage applied, and therefore the sensitivity to the disruption of the initial direction of alignment increases.
As described above, the uniformity of alignment is a very important factor for the IPS system, and the rubbing method currently employed has come to pose a problem. Generally, the alignment processing by rubbing harbors many problems such as the breakage of the TFT due to the electrostatic electricity generated by the friction, the display defects due to the disrupted alignment caused by the irregular thread ends of the rubbing cloth, dust or the frequent replacement of the rubbing cloth. For the purpose of obviating these problems of the rubbing alignment process, what is called “the rubbingless” alignment method has been studied in which the liquid crystal particles are aligned without rubbing, and has been proposed in various forms. The photo alignment method is one of the methods proposed, in which the polarized ultraviolet light is radiated on the surface of a high polymer film to align the liquid crystal particles without rubbing.
As an example, the method disclosed in “W. M. Gibbons et al., Nature, Vol. 351, pp 49-50 (1991)” has the feature in that the conventional rubbing process is not required, and the liquid crystal particles are aligned in a predetermined direction by radiation of polarized light. According to this optical alignment method, the problem of the scar on the film surface or the static electricity posed by the rubbing method is eliminated. Also, the fabrication process for industrial production is simpler. Thus, this method is closely watched as a new liquid crystal particle alignment method using no rubbing process.
The optical alignment method is roughly divided into the photo decomposition method and the photo reaction method. Both methods pose the following practical problems. Specifically, the material of the alignment layer requires some thickness to improve the hermeticity between the alignment layer material and the substrates. The provision of an alignment layer material having some thickness, however, makes it difficult to secure both light reactivity and transparency at the same time. In some cases, the coloring becomes so deep that the light utilization rate and the image quality are deteriorated.
In using the optical alignment method to a substrate in which the alignment layer has an underlying layer of a light-reflecting material, attention must be paid also to the path of the light reflected. The following subject is described in JP-A-2000-356776. The light radiated through the alignment layer for alignment process is reflected on a tapered surface of the electrode. This reflected light is reflected again on the lower substrate and enters the alignment layer again. Generally, the tapered surface of the electrode and the surface of the lower substrate are not always even. Therefore, the direction of polarization of the light radiated again on the alignment layer is different from the direction of polarization of the first light radiated on the alignment layer. In the case where the alignment layer material irradiated with the light polarized in other than predetermined direction develops the irregularities of alignment at the particular portion, thereby causing an alignment defect. To cope with this problem, JP-A-2000-356776 proposes the following configuration. Specifically, there is provided a liquid crystal display wherein a pair of transparent substrates are arranged in opposed relation to each other through a liquid crystal layer. An alignment layer subjected to the alignment process by radiation of polarized light is formed on the surface of at least one of the substrates. A plurality of metal electrodes are used to form an electric field in the liquid crystal layer formed on at least one of the transparent electrodes. Let a be the height of the step of the metal electrode, z the total distance of the side having a longer total step distance per pixel, θ the taper angle of the metal electrode, φ the angle of the incident light to the substrates, X the length of the side of one pixel along the row, Y the length of the side of one pixel along the column, and β the numerical aperture per pixel. The relation holds that a×z/(tan(θ+φ)×X×Y×β≧0.05 or more. Thus, the ratio of which the strength of the incident light radiated on the alignment layer, reflected on the metal electrode and radiated again on the alignment layer represents the strength of the incident light is at least 0.1. The alignment layer is subjected to the alignment process by the radiated light in which the tilt angle φ of the direction of polarization from the direction associated a larger aspect ratio of the step of the metal electrode is not more than 10 degrees. This configuration can produce a liquid crystal display which develops no disruption of the alignment layer and in which the reduction in contrast is suppressed.
With regard to the problem of light reflection in the case where the optical alignment method is used for the substrate having a layer of a light-reflecting material under the alignment layer, JP-A-2000-356776 describes only a path in which the light reflected on the tapered portion of the electrode is reflected again on the lower substrate and radiated again on the alignment layer but fails to refer to a case in which the light reflected on the tapered portion of the electrode directly enters the alignment layer. In solving the problem according this invention, a consideration is given also to a case in which the light reflected from the tapered portion of the metal electrode formed immediately below the alignment layer is directly radiated again on the alignment layer. According to this invention, as compared with JP-A-2000-356776 disclosing the case in which the light is reflected both on the electrode and the lower substrate, the reflected light has less chance of being absorbed and scattered and thus probably has a larger effect on the alignment layer.
Assuming that the optical alignment method is used for a substrate formed with a stepped structure substantially of the same height as the thickness of the alignment layer by the electrode adjoining the alignment layer and the IPS-TFT-LCD is test produced by use of the particular substrate, light leaks from the edge portion of the electrode. This light leakage causes an increased black level, and the resultant reduction in image quality such as a lower contrast ratio and lack of brightness uniformity. It has been revealed that this image quality deterioration is partly caused by the light reflected from the tapered portion of the metal electrode immediately below the alignment layer as described above. In the case where the taper angle θ of the end portion of the electrode is not more than 45 degrees as shown in FIG. 2, the polarized light reflected on the tapered portion of the electrode is again radiated on the part of the alignment layer surface 132 parallel to the substrates, and an area is created where the both the polarized light is radiated both directly and after reflection. Even in the case where the taper angle of the end portion of the electrode is larger than 45 degrees as shown in FIG. 3, an area is also created where the polarized light is also radiated depending on the thickness of the electrode or the thickness of the alignment layer. In view of this, the present inventors have made vigorous research efforts on a mechanism in which the polarization axis of the polarized light directly radiated and that of the reflected light are displaced from each other, and have found a new mechanism not described in JP-A-2000-356776. JP-A-2000-356776 contains the description that the tapered surface of the metal electrode and the surface of the lower substrate reflecting the polarized light are not uniform and therefore the polarization axis is displaced when the light is reflected from these surfaces. The mechanism of displacement of the polarization axis as found by the present inventors is as follows. Specifically, the IPS system basically requires the initial alignment by displacing the liquid crystal particles from the direction of electrode extension by at least a predetermined angle, and therefore the polarization axis of the light radiated is normally tilted by a predetermined angle with respect to the electrode. In this way, the polarization axis of the radiated light is displaced from the direction perpendicular or parallel to the direction of the electrode extension. Therefore, the polarization axis of the light directly radiated on the flat surface 132 of the alignment layer is different from the polarization axis of the light radiated again after being reflected on the alignment layer surface 132. As a result, the alignment layer surface 132 comes to have the ability to align the liquid crystal particles along two axes, thereby causing the alignment defects of the liquid crystal particles. In the IPS system in which the alignment irregularities are liable to be more conspicuous than in other systems, the alignment defects brings about the problem of light leakage. Also, since the electrode is normally formed of a metal material having a high refractive index, the light reflectivity of the boundary between the alignment layer and the electrode increases. Therefore, a special care must be taken of the reflected light. This is the case with both the double reflection on the lower substrate and the light reflection from a part of the tapered portion of the metal electrode immediately below the alignment layer.